The present invention relates to a system for cleaning thru holes in a printed circuit board and, more particularly, for repairing electrical short circuits on a printed circuit board.
The most rudimentary printed circuit board has a relatively stiff substrate, one surface of which supports an electrical circuit made of deposited conductive material, such as copper. Soon after single surface circuit boards were made and used, it became apparent that circuit density had to increase in order to adequately supply logic and power to all parts of more complex electronic systems. Accordingly, circuit board substrates were soon developed to support printed circuits on the upper as well as on the lower major substrate surfaces. The circuit planes were substantially parallel to one another.
As the sophistication of electrical circuits and printed circuit boards increased, designers began to consider electrically connecting selected terminals on an upper surface circuit to selected terminals on a lower surface circuit. In order to achieve this electrical connection between circuits on major surfaces of the board, viaduct holes (vias) were used. These vias or thru holes were drilled, punched or etched completely through the board, from the upper surface to the lower surface. Electrically conductive wires could then be inserted in the vias and connected to points or terminals on both circuits, thus forming the desired electrical interconnections. Alternatively, the walls of the thru holes could be plated with electrically conductive material to accomplish the same result. The latter technique has proved to be the more efficient and reliable for densely packed circuits with high resolution electrical lines.
As the sophistication of electronic circuitry continued to increase, multiple layers of circuitry were provided on, sometimes embedded in, one printed circuit board substrate. The latter technique is referred to as encapsulation. Thus, in addition to circuitry being provided on both major surfaces of the substrate, other layers of circuitry were disposed intermediate the major surfaces and substantially parallel thereto. In multiple layer circuit cards and boards, plated thru holes could still be used to electrically connect circuitry disposed on both major surfaces of the substrate.
In the course of manufacturing multilayer circuit boards, occasionally a short circuit occurs due to an intermediate conductive layer being inadvertently allowed to contact the plated wall portion of a via. Thus, although a point on a circuit located on the upper surface is designed to be electrically connected only to a point on a circuit located on the lower surface, for example, an intermediate layer of conductive material may short circuit this connection and cause anomalous operation, unexpected results or even serious damage.
Heretofore, repairing short circuits such as those hereinabove described required mechanical drilling of the thru holes to remove the conductive material with which their walls were plated. A non-conductive material was then introduced into the thru hole to fill part of the drilled area. The walls were then replated with conductive material to reestablish the desired electrical connection between circuits on the major surfaces of the substrate. Unfortunately, while this mechanical drilling process has generally proved to be effective in removing contaminants that cause short circuits, in the course of drilling, the diameter of the thru hole often increases. In order to ensure that all contaminants are removed, inevitably the thru holes are enlarged to a degree beyond that which is strictly required. Such over-enlargement often becomes unacceptable due to space limitations when hundreds of thru holes are provided on every square inch of circuit board surface.
Chemical processes for repairing short circuits near thru holes have heretofore not been generally used due to the difficulty in handling and controlling the flow of caustic etching compositions through relatively small diameter vias.
Vacuum systems have been used with etchants, but not for the purpose of cleaning thru holes and repairing electrical short circuits. U.S. Pat. Nos. 4,344,809 and 4,384,917, both issued to Wensink, teach the use of a jet etch apparatus and method for decapsulation of molded devices. An etchant solution flows through an etching block forming a jet spray that impinges upon a device to be decapsulated. The etchant solution is drawn through the etching block by means of suction created by a jet pump. Thus, epoxy is removed from an encapsulated chip but the electrical connections between the chip and printed circuits are not affected.
It would be advantageous to provide a system for chemically removing electrically conductive material from a thru hole.
It would also be advantageous for such a system to remove solder and copper from a plated thru hole without damage to adjacent networks.
It would further be advantageous to remove solder and copper from a plated thru hole without enlarging the hole.
It would further be advantageous to provide a system for chemically removing electrically conductive material from a plurality of thru holes simultaneously.
Moreover, it would be advantageous to repair short circuits that occur in multilayer printed circuit boards in the vicinity of thru holes.